After at least the half of the hydrochloric acid has been
boiled away, there are added at least five cubic centimeters nitric acid
of 1.2 specific gravity, partly to bring the iron to peroxide, partly to
destroy the organic matters formed from the carbon, which might possibly
be remaining and might tend to remove the color of the chameleon
solution. The boiling is now continued till near dryness, when five
cubic centimeters hydrochloric acid are added, after which the solution
is boiled as long as any reddish-yellow vapors of nitrous acid are
observed. When these have disappeared a drop of the liquid taken up on
a small glass rod is tested with an newly prepared solution of red
prussiate of potash (2 grammes in 100 cubic centimeters water), to
ascertain whether there is any protoxide of iron remaining. First, when
no indication of blue or green is visible, the test shows a pure yellow,
it is certain that there are no reducing substances in the solution.
If a trace of protoxide of iron remains in the solution another cubic
centimeter of nitric acid ought to be added and the boiling continued so
long as any reddish-yellow vapors are visible, more hydrochloric acid
also being added to keep the solution from being dried up.
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